The structure of a conventional differential device is shown in FIG. 12.
In FIG. 12 is shown a differential speed change mechanism 2 for differentially gearing right and left wheels, a pinion gear 3, a pinion shaft 4, and tapered roller bearings 50 and 60 for supporting the pinion shaft 4 so as to freely rotate, which are housed in a differential case 1. The pinion gear 3 is engaged with a ring gear 2a of the differential speed change mechanism 2 and is integral with the pinion shaft 4 at the inner end portion thereof. The pinion shaft 4 is supported, so as to freely rotate, on the inner side of the differential case 1 by the single row tapered roller bearings 50 and 60 of rear-surface-coupling type. At the outer end portion of the pinion shaft 4 is provided a companion flange 7, with which a propeller shaft (not shown) is coupled.
The tapered roller bearings 50 and 60 are respectively mounted to the inner peripheral surfaces of bearing-mounting annular walls 13 and 14 formed in a forged bearing case portion 1a of the differential case 1. The companion-flange-side tapered roller bearings 50 are installed from a small-diameter-side opening portion of the bearing case portion 1a. The pinion-gear-side tapered roller bearings 60 are installed from a large-diameter-side opening portion of the bearing case portion 1a. Spacers 8 for positioning are interposed between the tapered roller bearings 50 and 60. The tapered roller bearings 50 and 60 are fixed by screwing a nut 15 into the outer end portion of the pinion shaft 4 and fastening it into the companion flange 7, thereby applying a sufficient preload between the pinion gear 3 and the companion flange 7.
Lubrication oil is stored in the differential case 1 at a level L in the non-operating condition. The oil is splashed in response to the rotation of the gear ring 2a during operation, guided to the tapered roller bearings 50 and 60 through an oil introducing path 11 formed between the annular walls 13 and 14 in the bearing case portion 1a, and returned through an oil reflux path (not shown). Between the outer peripheral surface of the pinion shaft 4 on the outer end portion side thereof and the inner peripheral surface of the bearing case portion 1a are mounted oil seals 9 for preventing oil leakage, and further seal protection cups 10 for concealing the oil seals 9.
In the case of the conventional differential devices, a large frictional resistance resulting from a large thrust load acts on the pinion-gear-side tapered roller bearings 60. This leads to a larger rotational torque, which results in lowered efficiency of the differential device. Other drawbacks are: torque, on account of using oil for lubrication, increases due to a resistance caused by the agitated oil, it becomes necessary to provide the oil introducing path 11 and the oil reflux path in the differential case 1 resulting in the differential device of a larger size, and the bearing device, under the influence from foreign matter in the oil, undergoes a shortened life and requires maintenance.
Therefore, a main object of the present invention is to provide a bearing device for supporting a pinion shaft capable of reducing running torque to improve the efficiency of the device, suppressing the increase of the torque due to a resistance caused by agitated oil, reducing the device in size and weight, and improving the life of the bearing.